Medulloblastoma is one of the most common malignant brain tumors of children. To approach the problem of effective therapy, the development and progression of human medulloblastoma must first be understood. We hypothesize that the expression and shedding of gangliosides contributes significantly to the pathogenesis of medulloblastoma, and have (i) shown that shedding of structurally distinct tumor gangliosides characterizes human medulloblastoma as well as other tumors of neuroectodermal origin and that this ganglioside shedding occurs in both in vitro and in vivo into the cerebrospinal fluid, (ii) tracked the binding of shed tumor gangliosides to normal host cells (e.g. fibroblasts) found in the tumor microenvironment, and (iii) demonstrated marked inhibition of tumor formation by inhibition of tumor cell ganglioside synthesis through blockade of glucosylceramide synthase. These data combined with preliminary data showing an enhancing effect of gangliosides upon normal cell responses to growth factors, including proliferation and migration of endothelial cells, lead to the new Specific hypothesis that shed tumor gangliosides enhance the host angiogenic response to neuroectodermal tumors, thereby promoting tumor formation and progression. To test this hypothesis we will employ human medulloblastoma and murine melanoma cell lines. We will (i) conduct comprehensive in vitro studies to determine the effects of highly purified gangliosides from these tumor cells upon endothelial cell migration, proliferation, and vessel formation, (ii) determine the influence of membrane enrichment with tumor gangliosides upon associated signal transduction, (iii) develop tumor cell clones deficient in or lacking shed ganglioside by specific targeted antisense gene transfection of the key ganglioside biosynthetic enzyme, GM3 synthase and (iv) determine the effect of alteration of ganglioside synthesis and shedding upon in vitro assays of angiogenesis and in vivo tumor formation and angiogenesis. The results will establish definitively how shed gangliosides influence angiogenesis, elucidating a specific mechanism by which they influence tumor formation and regression and identifying a target for noncytotoxic therapeutic approaches to tumor progression.